1. The Field of the Invention
The present invention relates generally to apparatus and methods for curing fiber reinforced composite structures. More particularly, it concerns a system which circulates preheated, pressurized liquid through a pressure chamber in order to cure a composite structure.
2. The Background Art
Fiber reinforced composite parts have achieved widespread acceptance for use in the construction of articles and structural parts. Until recently, such applications were limited mainly to metal parts because of the conventional thinking that only structural metals were capable of withstanding the accompanying torsional and bending stresses. However, the development of fiber reinforced composites having high strength-to-weight ratios has resulted in composites being used in many structural parts, including ski poles, spars for hand gliders and the like, golf shafts, rocket launch tubes, mechanical drive shafts, and so forth.
Historically polyester resin systems with glass fibers were the first composites. They were hand layered into a mold and the resins were brushed onto each layer. Each subsequent layer was done this way until the desired thickness was attained. The polyester resins that were used were precatalyzed with a small percent of methyl ethyl ketone peroxide and this allowed the resins to generate the necessary heat to cause chemical cross linking or cure of the layers to occur over a long period of time at atmospheric pressure. The only pressure that the lay-up saw was the pressure applied by the technician during the lay-up and gravity. This usually resulted in layered parts that had resin-rich and resin-starved areas of the lay-up and inconsistent part to part qualities.
This hand lay-up process, however crude, is still used in this industry today but only for non-critical stress applications. There have been several improvements in the resin chemistry, and in the reinforcing fibers used. Even the methods used to cure have advanced somewhat by some manufacturers adding a vacuum bag over the parts to help improve product quality. However, this additional process is not cost effective when evaluated to the end product improvement values.
The lack of uniformly applied pressure during the curing process led to the development of the traditional autoclave oven-curing process. The autoclave oven is constructed with steel walls four to five inches thick, usually cylindrical in shape with a thicker walled steel door. The door requires the use of hydraulic rams to force the door locking ring mechanism to close and form an air tight seal. Internal electric or gas heating systems provide the source of heat and internal electric fans re-circulate the hot air inside the chamber during the cure cycle. The pressure of the cure chamber is raised by compressed air pressure, controlled by the autoclave operator. Pressures as high as 100 psi and temperatures of 350 degrees Fahrenheit and cure cycles that take 2 to 4 hours to complete are common practice with these devices.
The traditional autoclave oven is extremely dangerous to use because of the very large volume of air at high pressure needed to fill the working chamber. Larger volumes of high pressure gases are more likely to cause injury upon fracture of the vessel. The oven is extremely slow to heat up and to pressurize and slow to cool down. Even though this type of autoclave is popular in the industry, it is not very efficient to operate and the initial costs are extremely high.
Compacting of the composite part is typically accomplished by applying cellophane shrink tape over the surface of the part by a high pressure applicator, after which the part is cured in the autoclave oven. After the part is cured, the cellophane tape is removed, usually by a scrubber machine. The cellophane tape leaves creases in the part which must be smoothed out by sanding or grinding. The sanding and grinding often damages some of the reinforcing fibers which reduces the structural integrity of the part. Further, the cellophane tape cannot maintain a uniformpressure on the part because the high temperatures produced within the autoclave oven cause the tape to expand. Some of the compaction needed to prevent air voids and resin seepage is therefore lost during the curing process.